1. Field of the Invention
The present invention relates to a CPU mode switching circuit, particularly a CPU mode switching circuit enabling a CPU to be switched to a plurality of operation modes without adding a new terminal for switching the mode by utilizing an already existing input terminal.
2. Description of the Background Art
In general, a CPU can be operated in a plurality of operating modes and needs a proper mode switching to comply with an operation environment. For example, when a CPU is built in an IC card, a communication protocol of the IC card must fit in with that of a reader/writer, or reading/writing equipment of IC card for a good data communication. Accordingly, if the communication protocol of the IC card can be changed by switching the operation mode of the built-in CPU, the IC card can be applied to more number of readers/writers.
In order to switch a CPU operation mode for a certain purpose like corresponding to a plurality of communication protocols as described above, it is usual that a mode selecting signal is newly input to the IC card from the outside for selecting the CPU mode after reset besides an existing reset signal for resetting the CPU. In other words, the function of switching operation mode of the built-in CPU can be ensured by providing the IC card with a new input terminal for a CPU mode selecting signal in addition to existing terminals for power supply, grounding, data input/output and reset.
However, an IC card is usually thin and has only the minimum number of input/output terminals like 6 or 8 for example. It is not preferable to increase a terminal under such condition for allocating the CPU mode selecting signal besides basic input/output signals for the IC card, considering future possible expansion of terminals. If two operation modes are necessary, only one terminal is additionally necessary for the mode selecting signal. But if more operation modes are necessary, more terminals come to necessity, which may result in difficulty in keeping a thin form of the IC card.
Another problem is that a circuit generating the mode selecting signal is necessary to be added in a reader/writer connected to the IC card. Expansion of reader/writer's circuit and cost-up by this can not be prevented.